The present invention relates to a lighting panel device for illuminating a wide face such as a liquid crystal display.
A wide lighting panel device used as a front light or a back light for the liquid crystal display is well known. The lighting panel device comprises a light source, a lighting panel for introducing the light emitted from the light source at an end face of the plate, and a prism sheet disposed to face the lighting panel for radiating the light discharged from the lighting panel to an object.
FIG. 7 is a side view showing a conventional lighting panel device used as a back light of a liquid crystal display (hereinafter called LCD). The lighting panel device 20 comprises a light source 21 comprising a light emitting diode (LED), a lighting panel 22, a light diffusion sheet 23 opposite the front side of the lighting panel 22, a reflection sheet 26, a pair of prism sheets 24 and 25 each having a plurality of prisms 24a (25a). The prism 24a of the sheet 24 and prism 25a of the sheet are different from each other in prism extending direction. There is a difference of 90 degrees between both directions.
Referring to FIG. 8, the light rays emitted from the light source 21 enters the lighting panel 22 from an end face 22a and is reflected by a bottom saw teeth reflector face 22b. A light A discharges from the lighting panel 22 at a refraction angle θ with respect to a normal N. A light B is further reflected by an upper face 22c and bottom reflector 22b, and discharges from the lighting panel 22. A light C is discharged after repeatedly reflected in the lighting panel 22.
Light discharged from the lighting panel 22 is diffused by the diffusion sheet 23 and enters prism sheets 24 and 25. The light is refracted by prisms 24a and 25a and discharged from the prism sheets 24 and 25, arranged in parallel rays. The light emitted from the prism sheet 25 illuminates an LCD (not show).
The refraction angle θ is about 60°-80°, and the vertex of the prism 24 (25) is 90°.
FIG. 9 shows locus of light drawn by computer simulation of another conventional lighting panel device. In the device, the prism sheet 25 and diffusion sheet 23 are omitted. As shown in the drawing, the incidence angle of the light projected into an LCD 27 is large. The incidence angle is about 35°-40°. Such a large incidence angle may cause reflection of incident light, so that the illumination efficiency for the LCD reduces.